At present, in a mobile communication system, especially in a Time Division Duplex (TDD) system, a User Equipment keeps synchronous with the network side during data communication with the network side, in order to effectively receive and send data. In the existing system, for example, in the 3rd Generation Partnership Project Release 4 (3GPP R4) standard, synchronization is usually implemented via a dedicated channel in that: when no data is sent by a User Equipment or the network side for a long time, a special sequence is sent on the dedicated channel periodically for maintaining the synchronization; according to the special sequence sent by the User Equipment, the network side determines and notifies the User Equipment of a timing advance for transmitting data the next time, to assist the User Equipment to determine the subsequent transmission time and accomplish the synchronization process.
However, with increasing demands on a high-speed data service, technologies such as High Speed Downlink Packet Access (HSDPA) and High Speed Uplink Packet Access (HSUPA) are gradually introduced by the 3GPP in protocols of versions R5-R7, and downlink and uplink high-speed data services are provided over a shared channel, resources of which are centralizedly allocated by a base station scheduler of the network side.
Generally, when a user equipment does not experience data communication in a certain direction, the network side does not allocate any resource for the direction, and if a user equipment does not experience uplink and downlink data communication for a long time, the user equipment will be in an uplink out-of-synchronization state; at this point, when new data is generated, the User Equipment can perform normal data communication only if the User Equipment is re-synchronized. Specifically, when the HSDPA technology is employed in the downlink direction, the base station allocates downlink resources to the User Equipment; and after receiving data via the corresponding resources, the User Equipment decodes the data block and feeds back an ACK/NACK and information related to channel quality to the base station via an uplink control channel according to the decoding result; at this point, if the User Equipment is in the uplink out-of-synchronization state, the base station may consider the current transmission as failed because no feedback is received from the User Equipment, thus data retransmission is needed; as a result, a waste of system resources may be caused. Similarly, when the HSUPA technology is employed in the uplink direction, if uplink data transmission is performed without uplink synchronization, it is possible that the base station cannot receive the data correctly.
In view of the above problem, a method for keeping the User Equipment always in a synchronization state by periodically maintaining the uplink synchronization is usually employed in the prior art; however, for some services with a low activity coefficient such as a World Wide Web (WWW) service, it is possible that no data is sent for a long time, and in this case, it is less meaningful to maintain the synchronization of the User Equipment; further, the system resources may be wasted.
In addition, there exists still another method in the prior art, in which an enhanced random access process is introduced on the basis of HSUPA technologies, for reporting an uplink buffer state to the base station when the User Equipment has not been scheduled for a long time so as to get the synchronization of the User Equipment. Specifically, the User Equipment sends an enhanced random access sequence code to the base station, the base station determines timing advance information according to a signal delay received and feeds back an instruction to the User Equipment via a Fast Physical Access Channel (FPACH); after receiving the instruction from the base station, the User Equipment adjusts the transmission time of its uplink signal and notifies the base station of its buffer state via an E-DCH Random Access Uplink Control Channel (E-RUCCH), on which an uplink scheduling identifier E-RNTI (E-DCH Radio Network Temporary Identifier) of the User Equipment is carried. Therefore, when a User Equipment has uplink data for transmission and is in the out-of-synchronization state, the User Equipment is synchronized via an enhanced random access process before the subsequent data transmission.
However, in the current system, the HSDPA scheduler is located on an MAC-hs (Medium Access Layer for controlling the HSDPA) entity of the base station, and is responsible for the allocation of downlink resources and uses an HS-DSCH Radio Network Temporary Identifier (H-RNTI) to distinguish between user equipments; the scheduling of the HSUPA is accomplished by an MAC-e entity of the base station, and the user equipments are distinguished via an E-RNTI. The uplink scheduler and the downlink scheduler operate independently, and no communication exists therebetween. Thus, if there is data transmission in the downlink direction and the User Equipment is in the out-of-synchronization state, the synchronization cannot be obtained via the enhanced random access process. However, if a normal random access is employed to obtain the synchronization, a Radio Network Controller (RNC) is needed to forward the uplink information sent by the User Equipment on a Random Access Control Channel (RACH), so that the delay of the synchronization process is lengthened and the signaling overhead is increased.